feat(spells): Phase E.5 CastSpellRequest + Spellbook/enchantment state

Completes the client-side spell loop on top of Phase F.1. Player can
send cast requests; spellbook + active-enchantment state mirrors what
the server broadcasts.

Wire layer:
- CastSpellRequest (C→S, inside 0xF7B1 GameAction):
  - BuildUntargeted (0x0048): self-buffs, recalls, heal-self — 16 bytes.
  - BuildTargeted (0x004A): projectile attacks, target buffs/debuffs — 20 bytes.
- GameEvents parsers added:
  - 0x02C1 MagicUpdateSpell: spell-id → spellbook.
  - 0x01A8 MagicRemoveSpell.
  - 0x02C2 MagicUpdateEnchantment: spellId + layerId + duration + casterGuid
    (summary head; full stat-mod body deferred).
  - 0x02C3 MagicRemoveEnchantment: (layerId, spellId).
  - 0x02C7 MagicDispelEnchantment: same shape.

Core layer:
- Spellbook: learned-spell set + active-enchantment-by-layer dict
  with events (SpellLearned, SpellForgotten, EnchantmentAdded,
  EnchantmentRemoved). Duplicate learn is idempotent. Same-layer
  add refreshes duration. Purge fires per-record remove for UI
  cleanup.
- ActiveEnchantmentRecord: (SpellId, LayerId, Duration, CasterGuid).

Tests (10 new):
- CastSpellRequest untargeted (16 bytes) + targeted (20 bytes) wire encoding.
- GameEvents: MagicUpdateSpell, MagicUpdateEnchantment,
  MagicRemoveEnchantment round-trip.
- Spellbook: learn idempotent, forget, add/refresh enchantment,
  remove fires event, purge-all clears + fires per-record.

Build green, 555 tests pass (up from 544).

Ref: r01 §2 (wire casts), §3 (cast state machine), §5 (stacking rules).
Ref: r08 §3 opcodes 0x0048/0x004A, §4 opcodes 0x01A8/0x02C1-0x02C8.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/AcDream.Core/Spells/Spellbook.cs

@@ -0,0 +1,104 @@
+using System;
+using System.Collections.Concurrent;
+using System.Collections.Generic;
+
+namespace AcDream.Core.Spells;
+
+/// <summary>
+/// Client-side spellbook mirror. Tracks which spells the player has
+/// learned (<c>UpdateSpell</c> 0x02C1) + a parallel active-enchantment
+/// table keyed by layer id (<c>UpdateEnchantment</c> 0x02C2 etc.).
+///
+/// <para>
+/// The UI binds to the collection-changed events so the spellbook
+/// panel + active-buff bar redraw automatically when the server
+/// pushes changes.
+/// </para>
+/// </summary>
+public sealed class Spellbook
+{
+    private readonly HashSet<uint> _learnedSpells = new();
+    private readonly ConcurrentDictionary<uint, ActiveEnchantmentRecord> _activeByLayer = new();
+
+    /// <summary>Fires when a spell is added to the player's spellbook.</summary>
+    public event Action<uint>? SpellLearned;
+
+    /// <summary>Fires when a spell is removed (rare — usually on respec / admin).</summary>
+    public event Action<uint>? SpellForgotten;
+
+    /// <summary>Fires when an enchantment is added / refreshed.</summary>
+    public event Action<ActiveEnchantmentRecord>? EnchantmentAdded;
+
+    /// <summary>Fires when an enchantment is removed (expired / dispelled).</summary>
+    public event Action<ActiveEnchantmentRecord>? EnchantmentRemoved;
+
+    /// <summary>All currently learned spell ids.</summary>
+    public IReadOnlyCollection<uint> LearnedSpells => _learnedSpells;
+
+    /// <summary>All currently-active enchantments.</summary>
+    public IEnumerable<ActiveEnchantmentRecord> ActiveEnchantments => _activeByLayer.Values;
+
+    public int LearnedCount => _learnedSpells.Count;
+    public int ActiveCount  => _activeByLayer.Count;
+
+    public bool Knows(uint spellId) => _learnedSpells.Contains(spellId);
+
+    // ── Inbound handlers ─────────────────────────────────────────────────────
+
+    /// <summary>0x02C1 MagicUpdateSpell: learn a spell.</summary>
+    public void OnSpellLearned(uint spellId)
+    {
+        if (_learnedSpells.Add(spellId))
+            SpellLearned?.Invoke(spellId);
+    }
+
+    /// <summary>0x01A8 MagicRemoveSpell: forget a spell.</summary>
+    public void OnSpellForgotten(uint spellId)
+    {
+        if (_learnedSpells.Remove(spellId))
+            SpellForgotten?.Invoke(spellId);
+    }
+
+    /// <summary>0x02C2 MagicUpdateEnchantment: enchantment added / refreshed.</summary>
+    public void OnEnchantmentAdded(uint spellId, uint layerId, float duration, uint casterGuid)
+    {
+        var record = new ActiveEnchantmentRecord(spellId, layerId, duration, casterGuid);
+        _activeByLayer[layerId] = record;
+        EnchantmentAdded?.Invoke(record);
+    }
+
+    /// <summary>0x02C3 / 0x02C7 MagicRemove/DispelEnchantment.</summary>
+    public void OnEnchantmentRemoved(uint layerId, uint spellId)
+    {
+        if (_activeByLayer.TryRemove(layerId, out var record))
+            EnchantmentRemoved?.Invoke(record);
+        else
+            EnchantmentRemoved?.Invoke(new ActiveEnchantmentRecord(spellId, layerId, 0f, 0));
+    }
+
+    /// <summary>0x02C6 MagicPurgeEnchantments: clear all active buffs.</summary>
+    public void OnPurgeAll()
+    {
+        foreach (var rec in _activeByLayer.Values)
+            EnchantmentRemoved?.Invoke(rec);
+        _activeByLayer.Clear();
+    }
+
+    public void Clear()
+    {
+        _learnedSpells.Clear();
+        _activeByLayer.Clear();
+    }
+}
+
+/// <summary>
+/// Summary of one active enchantment layer on the player. Richer detail
+/// (stat mods, category, power) requires the full <see cref="ActiveBuff"/>
+/// struct — this record is the wire-slim version surfaced by the
+/// <see cref="Spellbook.EnchantmentAdded"/> event.
+/// </summary>
+public readonly record struct ActiveEnchantmentRecord(
+    uint SpellId,
+    uint LayerId,
+    float Duration,
+    uint CasterGuid);